Render Regions renders part of region only
Rendering a region will always have the margins added to it when completed. The purpose of this tool is to make it so you do not have to render an  entire image when you are test rendering, so the margins give you an idea of what part of the scene it is in. 

If you really need to use this method for a final render (NOT recommended, move your camera!) then you can use any image manipulation software to get the margins trimmed off. 

If you want to render different sections because of huge render times for the entire image, and then add them together in post-pro, that works well, too. 

HINT: Check transparent background settings, it will make your work a whole lot easier!



If this does not answer your question, please leave a comment, I'll try to get back to you.